1. Field of the Invention
This invention relates generally to low-friction nut constructions, and more particularly to improvements in the devices described and illustrated in U.S. Pat. No. 3,296,880 dated Jan. 10, 1967, issued to A. Maroth, and entitled RADIAL ROLLER ANTI-FRICTION TRANSMISSION.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
The above identified patent discloses a number of low-friction nut followers adapted for engagement with elongate screws of one type or another, or with splines. As generally shown in FIG. 8 of the patent, in such devices there is provided a nut body having radially extending holes which accept a series of rollers that engage the threads of a screw passing through the nut bore. Each roller (70) is turnably carried in a needle bearing (48). The rollers preferably are backed up by washer-like thrust ball bearings (50), which latter are held in position by disc-like threaded plugs (60a) screwed into the radial holes of the nut body.
Alternate constructions are illustrated in FIG. 7, showing a modified thrust bearing comprising a cage (54) and balls (52); and FIG. 9, which illustrates an arrangement where springs (83, 85) are employed between the threaded plugs and the roller elements (78, 82), in order to bias the latter inwardly and thereby tend to adjust the load on each element.
Other roller-carrying drive nuts are shown in U.S. Pat. No. 3,779,094 dated Dec. 18, 1973, issued to B. LaBarre, and entitled REVERSING NUT FOR A DIAMOND THREAD SCREW; and U.S. Pat. No. 3,792,616 dated Feb. 19, 1974, issued to M. Wentzel, and entitled RECIPROCATING DRIVE. In U.S. Pat. No. '616, radially disposed rollers are carried by means of bushings (40). Each roller has a pair of needle bearings and an end thrust bearing, together with a pair of back-to-back springs (36) which normally bias the rollers forward into engagement with the screw threads, but which can yield outwardly an extent in the event a particular roller experiences an excessive radially outward force. The end result was a tendency for the springs to equalize the forces applied to each roller. In the disclosed device the nut was employed with a screw shaft having a pair of aligned closed loop-like grooves in which the rollers were received, so that the resulting motion of the nut was to reciprocate as the shaft was turned.
U.S. Pat. No. '094 discloses a reversing nut construction having a nut body with radially extending holes in which there is received a series of radially extending rollers, for engagement with the threads of a diamond thread shaft. Again, there are employed radial bushings as housings for the respective rollers, and a pair of needle bearings is interposed between each roller and the wall of its bushing. Thrust bearings are located at the outermost end of each roller.
Still other types of nut follower devices are disclosed in U.S. Pat. No. 4,031,765, dated Jun. 28, 1977, issued to R. Metz, and entitled REVERSING NUT FOR A DIAMOND THREAD SCREW; and U.S. Pat. No. 4,198,872, dated Apr. 22, 1980, issued to R. Metz, and entitled MECHANICAL SCREW TRANSMISSION. The latter two patents employed balls as roller elements between a nut body and suitably threaded shaft. U.S. Pat. No. '765 utilized spring rings (75, 76) to restrain the balls and bias them into engagement with the screw threads. U.S. Pat. No. '872 shows a bowed spring retainer washer (46) to bias each ball-carrying bushing in a radially inward direction.
In all of the disclosed devices provision was made to permit limited radially inward/outward shifting of the balls/roller elements as dictated by different requirement of absolute load on the nut body, and of relative changes in load between the balls/rollers, in an effort to equalize or share the load. The disclosed constructions had the advantage that slight inconsistencies in the thread pitch from one point to another could be accommodated as a consequence of the ability of the balls/rollers to move inwardly or outwardly.
In spite of considerable commercial success of several of the devices noted above, it has been felt that there was some room for improvement in manufacturing and assembly procedure, as well as reduction in costs involved therewith.
In particular, the device disclosed in FIG. 8 of U.S. Pat. No. '880 was rather expensive to manufacture and produce, due to the relatively large number of separate components involved, namely a roller, a needle bearing, thrust washer (74), a thrust bearing (50), and threaded retainer plug (60a). Further, the adjustment of each plug in order to establish a predetermined, desired radial position of each roller was often difficult to accomplish in practice. Adjustment of one plug and roller often upset prior adjustments to the others, rendering an absolute "balance" condition of each roller extremely difficult to achieve. As a result, in the practice of the invention, one or several rollers often carried more than their relative share of the load. The large number of individual parts, coupled with the likelihood to have to resort largely on manual assembly and adjustment, made the devices fairly expensive and time-consuming to produce on a large scale.
A further consideration was the fact that the tolerances of the needle bearings which were available were generally poor, and hence the need arose for spring-biasing of the rollers in order to compensate for such poor tolerances.